In the airport, how did Cady know... (MINOR SPOILERS)

...after such a brief conversation with the airline employee, that Bowden did not get on the flight? I've watched it twice, and I just don't see it.

Or did Cady believe that Bowden did get on the flight? His behaviour afterwards suggested to me that he didn't believe it.

The script doesn't tell you much:
"- I have a brief here for Sam Bowden.
- Yes, sir.
- Was he on that flight 403?
- Yes, sir, he was.
- Now I don't know whether to mail it to him or not.
- You don't have any idea when he'll be back, do you?
- Let's see. He did make a return reservation, sir.
- 6:00 p.m. On Thursday.
- Then I guess special delivery will do.
- Thank you.
- You're welcome.".